Designers at the Royal College of Art have created a floating black sphere that, in addition to stalking people, records and replays sounds it picks up after a short delay.

It's called "Space Replay" and it was designed by Francesco Tacchini, Julinka Ebhardt and Will Yates-Johnson. The intention was to create a hovering object that explores and manipulates transitional public spaces with particular acoustic properties. "By constantly recording and replaying these ambient sounds," the designers write, "the levitating sphere produces a delayed echo of human activity."

In the video, the sphere can be seen moving through a subway station and following people directly into an elevator — all while playing back recently recorded audio. It makes for a particularly eerie effect.

Incredibly, no special effects were used to create the video. The black balloon was filled with enough helium to lift its internal components and hover around at its buoyancy point. In all, the device weighed 120g (4.2 oz).

For the recording and playback, it was equipped with a battery-powered Arduino — an Adafruit Wave Shield the designers hacked to record and playback audio on-the-fly, along with a small speaker. These were inserted into a latex balloon after it was shaped into a cone, which enhanced the sound coming from the speaker while protecting the balloon from the wires and PCB edges.